Enhancement of second-harmonic generation through Brillouin zone folding in a waveguide-coupled metasurface.

Metasurfaces have become significant platforms for optical manipulation, yet unlocking their full potential for nonlinear optics requires novel mechanisms to control and enable frequency conversion processes. This study demonstrates how structural dimerization in plasmonic metasurfaces coupled to waveguides can modify linear and nonlinear optical behavior via Brillouin zone folding. By introducing a centrosymmetric unit cell design featuring two mirrored split-ring resonators, we allow guided modes that were previously below the light line to appear as guided-mode resonances. These resonances facilitate nonlocal modes, which are present as distinct narrow transparency windows. Although centrosymmetric dimerized design typically forbids far-field radiation through quadratic nonlinear interactions, we observe notable second-harmonic generation - not merely through symmetry breaking at oblique incidence, which proves insufficient, but rather with the support of a nonlocal mode. The excitation of a collective mode at the pump frequency provides a strong nonlinear response by mediating the formation of a net dipole moment at the second-harmonic frequency, enabling far-field radiation that is otherwise forbidden. This synchronized action among split-ring resonators leads to observable second-harmonic generation, confirmed by both experimental evidence and simulations. Our results indicate that dimerized metasurfaces represent a versatile platform for harnessing collective modes in nonlinear interactions. This motivates further research and suggests promising applications in advanced nonlinear photonic devices.